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Cold-curing epoxy resins: Aging and environmental effects. II - Mechanical properties
No full textThermosetting adhesives, suitable to join different materials in structural applications, are influenced during their service life by various effects which might cause them damage and that are not deeply experienced. They can be, in fact, exposed to atmospheric influences as well as subjected to physical aging as a consequence of their glassy non-equilibrium nature.
In the first part of this study, the influences of natural exposure and physical aging, occurring separately or concurrently, on thermal properties of cold-curing epoxy adhesives have been examined. This second part analyzes weathering influences on the mechanical properties of the same adhesives. For this purpose, the different aging processes have been first separated, in order to investigate any single effect, and then their simultaneous influences have been evaluated.
The specimens only physically aged exhibit the highest values of static modulus and yield strength, in accordance with current literature. The exposure to natural weathering (i.e. to chemical and physical agings) determines a fluctuation of mechanical properties, whose average values, however, remain close to the initial ones. The effects of chemical aging have been found particularly severe on the filled adhesive. A de-aging treatment, able to erase only physical aging and to remove a limited amount of sorbed water, results in an incomplete recovery in modulus and strength. This effect is partially due to the crosslinking reactions taking place in the cold-cured adhesives.
It is thought that the results of this qualitative study can be extended to longer periods of natural exposure, even in the absence of any critical external agent, and that the behavior observed for the selected adhesives can be considered comparable to that of other cold-curing epoxy adhesives
Epoxy resins employed in civil engineering applications: effects of exposure to mild temperatures
No full textAdesivi Epossidici nel Restauro del Calcestruzzo: Esposizione Artificiale e Naturale agli Agenti Atmosferici – Effetti sull’Adesivo
No full textCaratterizzazione di Miscele Compatibili di un Polimero Semicristallino (Polietereterchetone) con un Polimero Amorfo (Polieterimide)
No full textCold-Curing Epoxy Resins: Aging and Environmental Effects. I – Thermal Properties
No full textThe effects of physical and chemical aging have been investigated on two cold-curing epoxy adhesives.
One of the two has been physically aged at different temperatures below its glass transition (i.e. 10, 23, 30 and 40°C) and then subjected to enthalpy relaxation measurements in a differential scanning calorimeter (DSC). According to recent literature, DSC studies have suggested that enthalpy relaxation gradually increases with aging time to a limiting value, depending on the temperature, until structural equilibrium is reached.
Specimens of both adhesives, previously cured at room temperature for 10 days, have been aged in dry-dark atmosphere or in a freezer (–20°C) for a prolonged time, measuring Tg and enthalpy relaxation at different time spans. When the structural equilibrium is reached, the samples aged in the dark-dry atmosphere exhibit Tg and relaxation peak values similar to those found in the earlier analysis for samples isothermally cured at 23°C. Samples aged at –20°C, the curing time suggested by suppliers which was not sufficient to cure the adhesives completely, have glass transition temperatures that are always lower.
The cured adhesives have also been exposed to natural weather for up to 36 months. Some samples, taken at different periods of time, have been analyzed in DSC. Other samples have been subjected to a de-aging procedure (24 hours at 50°C) before the thermal analysis. The natural exposition, regarded as chemical aging, determines a cycling change of Tg of both adhesives around average values slightly lower than the initial ones. This behavior cannot be erased by the de-aging procedure, which is able to delete only the effects due to physical aging. Enthalpy relaxation peaks, characteristics of physical aging, are observed only for exposed samples not subjected to de-aging procedure. These samples, finally, are subjected during the natural exposition to aging and de-aging processes taking place in non-isothermal conditions and, therefore, not predictable
Prodotti Epossidici utilizzati nel Recupero di Sistemi Strutturali: Interazioni con l’Ambiente
No full text
Crystallization Behaviour and Mechanical Properties in Blends of Semicrystalline (PEEK) and Amorphous (PEI) High Performance Engineering Thermoplastic Polymers
No full textCrystallinity and mechanical properties of blends with different amounts of semicrystalline poly(aryl/ ether ether ketone) (PEEK) and amorphous poly(ether imide) (PEI) polymers have been studied. The blends, prepared by melt mixing, have been investigated by differential scanning calorimeter (DSC)to analyze the miscibility
between the components and the final crystalline content. Moreover, for the 20/80 PEEK/PEI blend, crystallization in dynamic and isothermal conditions has been carefully investigated in order to find proper conditions for maximum development of crystallinity. Mechanical tests (static and dynamic) have been performed
to evaluate the properties of the as-molded and crystallized blends and to compare them with those of crystalline PEEK and amorphous PEI neat resins. Finally, a few SEM observations have been performed to compare the fractured surface of the blend with those of the pure constituents
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